Probing the Electronic Structure of HfO2 polymorphs via Electron Energy Loss Spectroscopy
P. Rauwel1,2,* and E. Rauwel3 1 Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia 2 Center for Materials Science and Nanotechnology, Dept. of Physics, University of Oslo, Sem Saelandsvei 26, NO-0316 Oslo, Norway
3 Dept. of Sustainable Energy, Tallinn University of Technology, Tartu College, Puiestee 78, 51008 Tartu, Estonia * Corresponding author: [email protected]
Hafnia is one of the most widely used wide gap materials in the microelectronic industry. Stemming from the need to miniaturize electronic devices, high κ materials such as hafnia-based compounds replace SiO2 in gate oxides, thereby decreasing the equivalent oxide thickness of the gate dielectric. On the other hand, scaling down deteriorates the physical properties of the latter viz., leakage current and carrier mobility. By increasing κ or the dielectric constant of the material, these losses can nevertheless be compensated for. Doping of HfO2 is known to increase its κ; these dopants include mostly divalent elements such as Mg or trivalent elements such as Y, Al and La. Multivalent elements such as V and Ta are also used as dopants but their oxidation state depends upon the growth conditions. Doping via elements with +2 and +3 oxidation states, has a propensity to introduce oxygen vacancies and stabilize higher symmetry phases of HfO2 i.e. cubic, tetragonal and even orthorhombic which present higher κ compared to the monoclinic phase. In all cases, the electronic structure of the material at the atomic level, varies as a function of crystallinity, crystal structure, oxygen vacancies and dopants; all of which affect bond lengths and in turn the atomic bonding and thereby the hybridization. Moreover, the electronic properties also affect the physical properties of the gate oxide and therefore, understanding the oxidation state of the dopants which affect the electronic structure of HfO2 by probing the O-K edge Energy Loss Near Edge Structure (ELNES) of the material via Electron Energy Loss Spectroscopy (EELS) in the Scanning/Transmission Electron Microscope (STEM), is indispensable. In fact, by using techniques such as Spectral Imaging (SI) and Energy Filtered Transmission Electron Microscopy (EFTEM), it is now tractable to study the diffusion of dopants within HfO2. When EFTEM is coupled with (ELNES), local changes in the electronic structure as a function of the concentration of dopants are revealed.
For over 6 decades, the microelectronic industry had been using SiO2 extensively as a gate oxide. However, SiO2 offers limited scaling down which in turn, has proven to be detrimental to its physical properties as a gate material, viz., leakage current, oxide breakdown and channel mobility [1]. One way of overcoming these problems of miniaturization required substituting silicon-based gate oxides by a thicker oxide layer with a higher κ such as hafnia based compounds which today have already been integrated into new generation transistors [2]. In fact, there is a number used to compare the performance of high-k dielectric Metal Oxide Semiconductor (MOS) gates with the performance of SiO2 based MOS gates. This corresponds to the thickness of SiO2 gate oxide, needed to obtain the same gate capacitance as the one obtained with a higher dielectric constant gate oxide thicker than the SiO2 dielectric; e.g. Equivalent oxide thickness (EOT) of 1 nm (1nm of SiO2) would result from the use a 10 nm thick dielectric featuring κ = 39 (k of SiO2 is 3.9). Thus, many efforts in doping HfO2 to obtain higher symmetry polymorphs, such as cubic, orthorhombic or tetragonal phases, presenting higher κ were made. In fact, HfO2 has four polymorphs: cubic, tetragonal, orthorhombic and monoclinic. Ab initio calculations suggest that high symmetry phases of HfO2 viz. cubic, tetragonal and orthorhombic have a higher dielectric constant (κ>25) than the monoclinic phase (κ~16-20) [3, 4]. It is therefore desirable to stabilize a higher symmetry phase of HfO2 which in fact, also turns out to be a challenge as it only stabilizes at very high temperatures i.e. 2700 ̊C for the bulk cubic polymorph. Nevertheless, other methods to stabilize the cubic phase via doping, by including a cation into the solid solution, have been proposed. In effect, divalent and trivalent dopants introduce oxygen vacancies into the structure of HfO2 which in turn, stabilize a high temperature phase of HfO2 such as the cubic phase [5-7]. Some of the well-known dopants to stabilize the cubic phase of HfO2 include CaO, CeO2, MgO and Y2O3 [8]. In fact, the latter three crystallize into the fluorite cubic structure and when used as dopants, compel HfO2 to stabilize also into its inherent fluorite cubic structure. Other dopants such as Dy, Sc, Al, have also been used to stabilize the cubic phase [9, 10]. Similarly, the tetragonal phase of HfO2 has been stabilized by Al2O3 [11, 12]. Recently, it was demonstrated that it is also possible to stabilize the cubic phase of HfO2 via oxygen vacancies alone. This requires synthesis techniques with reductive solvents and precursors that induce oxygen vacancies into the structure of HfO2 which has proven to be instrumental in stabilizing the cubic phase of HfO2 [13]. Several Transmission Electron Microscopy (TEM) characterization techniques have been employed for HfO2: High Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM), High
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Angle Annular Dark Field Microscopy (HAADF), Energy dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS) and Energy Filtered Transmission Electron Microscopy (EFTEM) [14]. This is because TEM is an indispensable tool in today’s nano-world where size-dependent physical properties of materials are being scrutinized in order to push the limits of miniaturization and such is the case of HfO2 [15-17]. EELS essentially provides the energy distribution of electrons transmitted through a sample. Valence electron energy-loss spectroscopy (VEELS) provides both high energy and high spatial resolution suited for band gap measurements, particularly in microelectronic devices that are HfO2 based. The energy spread of the electron source is important for VEELS as the tail of the zero-loss peak hinders easy signal-background separation. Recently, several types of monochromators were developed and advantages of using monochromated EELS were also demonstrated especially in determining the band gap of materials and studying the fine structures in the low loss signal [18]. EELS works on the principle of energy loss of the incoming electrons in a (S)TEM where some of the incoming electrons will have lost energy through inelastic scattering in the sample, on exciting core electrons from their ground state. These excitations involve transitions of electrons from the occupied core levels into empty states above the Fermi level. Since the initial core levels are highly localized in energy, these transitions following the dipole selection rules are mainly sensitive to final-state effects. The occupancy of these final states may change due to bonding through charge transfer or hybridization, and this will be reflected in the shape of the Energy Loss Near Edge Structure (ELNES). The empty states available are determined to a good approximation by a dipole matrix element between the initial state of the atomic electron, which is localized on the excited atom and the final unoccupied state. Thus, using ELNES in EELS, we probe the local density of empty states of the material and obtain information on the bonding and the local chemistry. O-K edge ELNES provides qualitative differences between cubic, tetragonal and monoclinic phases of HfO2 due to charge transfer and hybridization effects, in turn, causing a change in the d-character of the valence electrons [19]. The energy loss mechanism of excitation of the 1s electrons to the 2p-orbitals respecting the selection rules, provides direct access to the density of the O 2p state. An EEL spectrum is typically divided into three parts: no loss (or zero loss region), low loss region and the high loss region. The ‘No’ loss and low loss regions provide the energy resolution of the spectrometer, optical properties and the plasmon interactions within HfO2. The high loss or the core loss region on the other hand, provides access to ionization edges representing high energy electron interactions with the core electrons and exhibits atomic bonding and molecular hybridization in the material. The development of aberration corrected (Cs corrected) microscopes [20] combined with high intensity electron sources and monochromators, allow sub-Ångstrom spatial resolution and an EEL spectrometer energy resolution of 0.15 eV that is consistently improving [21, 22]. This present chapter culminates various EELS studies of HfO2 conducted by various groups. The focus of this chapter remains on the electronic structure of HfO2 and their variations due to crystallinity changes, dopants and presence or absence of oxygen vacancies. The deviation of the chemical structure from one of the ideal HfO2 crystalline structures is clearly visible through the shape and energy localized O-K edge ELNES spectra which provide a great amount of knowledge on the bonding and hybridization at the atomic scale. On the same note, dopants that modify the electronic structure undergo diffusion in HfO2 and can be quite well studied at the nanometer scale using EEL signals via Spectral imaging (SI) and EFTEM. In fact, each metallic dopant presents a different diffusion dynamic and its influence on the electronic structure of HfO2 deserves attention.
2. Electronic structure of undoped Hafnia
2.1 VEELS of pure Hafnia
In this section we will take a closer look at the low loss and high loss regions of the EEL spectra of pure hafnia. In fact, low loss EELS of HfO2 or VEELS, provides information on the band gap, plasmon peak, single electron transitions and Hf-O interactions. Agustin et al. studied the effect of annealing on the electronic structure of HfO2 grown on Si substrate via Atomic Layer Deposition (ALD) with TiN electrode grown on top of the HfO2 film, using an EEL spectrometer with energy resolution of 0.85-0.9 eV [23]. Figure 1 presents the VEELS of the as deposited sample pure HfO2 which was further annealed at different temperatures. There a various peaks labeled in the figure: peak A corresponds to the band gap, peak B is a single electron transition between O 2p and Hf 5d states, peak D corresponds to the plasmon losses; while as, the other peaks correspond to the Hf-O edges. The as grown film labeled ‘as depose’ in fig. 1, presents broad peaks in the VEEL spectrum which take shape on annealing; sharper peaks appear mainly due to crystallization of the films, reduction of point defects and elimination of hydroxyl groups that exist during HfO2 thin film deposition, a very common problem with ALD precursors along with using water as an oxidizing source. In fact, broader peaks were due to point defects as well as amorphous HfO2 that damp certain electronic transitions and broaden the signals in the VEEL spectra. This damping was also noticeable in the films annealed up to 700 ̊C and sharper peaks appeared only at annealing temperatures of 800 ̊C and higher. Compared to the bulk reference VEEL peak, even the films annealed at 900 ̊C, did not produce similar peak shapes and intensities implying that nanostructuring of HfO2 could be plausible which in turn, would damp certain volume plasmons. Furthermore, point defects such as oxygen vacancies are very common with high temperature processing of HfO2, thereupon producing changes in the electronic
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structure. All the same, the presence of oxygen vacancies can induce defect states in the band gap and the onsets of these peaks are only visible with high energy resolution spectrometers with full width at half maximum of 0.15eV or higher. Nevertheless, the onset of the band gap peak A also improved with annealing; this was believed to be due to the TiN electrode which acted as a sink for point defects.
Efforts in determining the band gap were also carried out by Cheynet et al. [24], who used a little higher energy resolution EEL spectrometer of 0.5eV, along with spectral noise reduction via peak fitting (fig.2(a)) and Rafferty Brown (fig.2(b)) methods [25] and obtained a band gap value of 5.3±0.5eV; Hafnia´s band gap depends indubitably heavily upon the crystal structure and defects in the films. In their study, HfO2 was grown by ALD and annealed at 450 ̊C and 500 ̊C, thereupon low loss EELS was performed. It appears that annealing them at different temperatures stabilized different phases viz., Orthorhombic and Monoclinic. Literature results on band gap, give different values for different phases ranging from 5.52eV to 5.8eV depending upon the phase of HfO2 stabilized [26, 27]. In the case of EELS band gap determination, a probe size of 0.25 nm was used corresponding to approximately 5 HfO2 atomic unit cells, therefore the band gap was probed locally, compared to other known optical techniques that probe the entire volume of the sample for band gap determination. Moreover, the band gap over the sample would probably show variations in the VEEL spectrum also. HfO2 has been known to have a direct and indirect band gap [28] and their studies have shown a direct band gap with a few indirect contributions.
Fig. 2 Band gap determination of the HfO2 layer in the Si/SiO2/HfO2 /poly-Ge based on (a) the fitting method and (b) the Rafferty-Brown method. Cheynet et al. J. App. Phys. 2007 [24]. Copyright license 3401820718149.
2.2 O-K Edge ELNES of Hafnia polymorphs
Even though the VEEL spectrum is capable of giving us information on single electron transitions, variations in surface and volume plasmons and band gap of the material as a function of various defects and phases of HfO2, information on the local chemistry of the different polymorphs on the other hand, can only be obtained from the O-K edge ELNES. Mizoguchi et al. [29] have carried out partial density of states (pDOS) calculations on the O-K edge of HfO2 for three polymorphs: cubic, tetragonal and monoclinic. Comparison of the 3 polymorphs in fig.3, shows differences in peak positions. In fact, peaks A, B, C, D are hybridization of the O 2p component with Hf deg, dt2g, s and p components, respectively. Peaks A and B are called doublets. Moreover, the pDOS calculations show that the cubic (fig.3 left) and tetragonal (fig.3 middle) ELNES spectra are rather similar, except for certain amount of broadening in the tetragonal
Fig. 1 Low-loss EELS recorded from the middle of the HfO2 films annealed at different temperatures and from the HfO2 reference powder. Significant peaks are labeled A–H. ~5.6 eV(A), ~15.5 eV(B), ~19 eV(C), ~26.6 eV(D), ~35 eV€, ~37 eV(F), ~42 eV(G), and ~47 eV(H). Agustin et al. Appl. Phys. Lett. 2005 [23]. Copyright license 3401820640464.
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ELNES fine structure. The monoclinic structure (fig. 3 right) on the other hand, shows clear broadening for peaks A and B compared to the other 2 polymorphs. The differences in the ELNES have been ascribed to the local Oh symmetry. In fact, in the cubic structure, oxygen is co-ordinated to 4 Hf atoms and places itself at the center of a regular tetrahedron while as, in the monoclinic structure, there are 2 types of local co-ordinations: O linked to 3 Hf atoms and O linked to 4 Hf atoms in a distorted tetrahedron, giving a total co-ordination of O7. These two different oxygen co-ordinations broaden the peaks in the ELNES of the monoclinic phase which will be described below. In the tetragonal structure, the co-ordination is O8 even though the tetrahedron containing the O atom in the center, is not a regular one, thereupon slight differences with respect to the cubic HfO2 ELNES appear. This further goes to reiterate that ELNES is capable of discerning local chemical structures. However, there are no literature reports of experimental results stabilizing the orthorhombic or tetragonal phase of HfO2 alone.
Fig .3 O K ELNES (above) and PDOS (below) of (left) cubic, (middle) tetragonal, and (right) monoclinic HfO2. Mizoguchi et al. J. Phys: Condens. Mater., 2009 [29]. Reproduced with permission from IOP publishing.
Fig. 4 Calculated ELNES of (A) O4 coordinated, (B) O3 coordinated, (C) combined O3 and O4 coordination, and (D) spectra C corrected for a FWHM of 0.7eV. Experimental ELNES spectra obtained for (E) the monoclinic phase and (F) cubic phase of HfO2. Atom color codes: blue is Hf and red is O. Rauwel et al. J. Appl. Phys. 2012 [13]. Copyright license 3401820820277. In 2012 Rauwel et al. stabilized the cubic phase of HfO2 [13] without dopants in a one pot synthesis method and experimentally obtained the first ELNES fingerprint for the pure cubic HfO2 phase (fig. 4(F)) and further elucidated the O-K ELNES spectra for the monoclinic phase (fig. 4(E)) via First Principles Calculation within the Density Functional Theory. Furthermore, the results obtained agreed very well with the theoretical ELNES spectra of Mizoguchi et al. Further analysis of the Calculated ELNES spectra of the monoclinic phase, seconded the broadening of the peaks calculated by Mizoguchi et al. [29] which in fact corresponds to a split in the peaks which is very clearly observable for
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peak A (fig. 4 (A) and (B)). This occurs due to different energy localizations of the 1s states of O3 and O4 co-ordinated Hf. In fact, the difference of 0.51eV in the 1s state of the 2 co-ordinations, is retained in the transitions from the 1s state to 2p state for both the co-ordinations, whereby producing two distinct ‘A’ peaks for each coordination. On summing up the 2 peaks, we obtain a broadened A peak for the entire monoclinic structure (fig. 4(C)). These differences in co-ordination are further manifested in the C and D peaks which also experience an energy shift due to the two different oxygen co-ordinations. In the experimental ELNES spectrum this split is not visible due to the energy resolution of 0.6 eV EEL spectrometer. Split in the peak can be however detected with a higher energy resolution of 0.15eV.
2.3 Effect of Oxygen Vacancies on the Electronic structure of HfO2
Point defects such as interstitials and vacancies affect the electronic structure of HfO2. The low loss spectra studied by Augustin et al. [23] also shows contribution from oxygen vacancies. In fact, oxygen vacancies cannot be neglected and are induced either during growth due to under-oxygenation of HfO2 or on annealing the sample, and will inevitably have an effect on the chemical structure. Just the same, amorphous contribution of HfO2 also plays a role on the electronic structure and affects the local bonding and hybridization which then are manifested as differences in the shape and energy localizations of the fine structure of the ELNES spectrum. For films grown by ALD, the effect of annealing on the electronic structure was studied by Wilk et al. [30] by exploiting the O-K edge for each annealing condition as manifested in fig. 5. Additionally, they observed a sharpening of the doublet from annealing temperatures of 600 ̊C onwards up to 1000 ̊C (fig. 5, spectra a-d) where the doublets become very prominent. The ‘as grown’ film was amorphous and the annealed films where monoclinic as deduced from their HRTEM studies. They attributed the sharpening of the doublet to the increase of the local Hf co-ordination owing to the increased crystallization on increasing annealing temperature. They further viewed this as a reduction in the concentration of point defects viz., oxygen vacancies and H substitution on O sites, deriving from the ALD growth process [31]. In effect, a well ordered crystal structure is necessary for good electrical properties of HfO2 and high annealing temperatures helped in achieving good quality films as shown by the O-K ELNES. This is a good example of co-relating the microstructure and electronic properties to the electrical properties of HfO2.
The effect of oxygen content during growth on the electronic structure of HfO2 was further studied by Jang et al.
[32]. They considered both the high loss and the low loss spectra in their study. Their samples were also grown by ALD using Ozone as an oxidation source. Two types of films were used in their study, depending on the oxidation time of Ozone: high oxygen content film (O3 5s) and low oxygen content film (O3 1s). They also used stoichiometric HfO2 powders as a reference. Another reference sample consisted of depositing Hf metal on a Si substrate and subsequently oxidizing it in air. HRTEM studies showed that both the ALD grown films were amorphous. In figure 6, VEEL spectrum of low and high O content HfO2, show very different profiles. In fact, VEEL spectrum of the high O content (O3 5s) HfO2 (fig. 6 red dashed line) is similar to the bulk HfO2 low loss spectrum (not shown here), while as the peaks in the low oxygen content (O3 1s) VEEL spectrum (fig. 6 black solid line) show rather broad features indicative of damped transitions, most certainly due to point defects and amorphousness and resembles the as deposited sample of fig. 1. On the other hand, the low oxygen content film on annealing (fig.7 (a) red curve) shows a broader first peak of the doublet than the high oxygen content film (fig. 7 (b) red curve) in the O-K edge ELNES. This implies that the oxygen poor HfO2 film possesses a more defective structure; whereas, the oxygen rich HfO2 has an electronic structure that resembles bulk HfO2. The first peak of the doublet in the as grown low oxygen content film (fig. 7(a), black solid line), is sharper than in the high oxygen content films (fig. 7(b) black solid line). On the other hand, the O-K ELNES for the low O content films nevertheless broadened on annealing (fig. 7(a), red line); while as, the peaks of the high oxygen content films, sharpened (fig. 7(b), red line). HRTEM and electron diffraction studies on the post-deposition annealed (PDA) films suggest the monoclinic phases for both samples. However, the low oxygen content film on recrystallization produced more metastable higher symmetry phases of HfO2 which have been studied by the authors through electron diffraction, certainly due to large number of oxygen vacancies. The microstructure and the electronic properties of the low oxygen content films, improved only very little compared to the high oxygen content film, suggesting that the
Fig. 5 O–K edge fine structure in HfO2 films after different annealing regimes: (a) as deposited, (b) 600 °C for 30 s (c) 850 ̊ᵒC, and (d) 1000°C annealed. Spectra (b)–(d) were recorded from single grains of the monoclinic phase in a zone-axis orientation; (a) was amorphous. There is a systematic sharpening. Wilk et al. App. Phys. Lett. 2003 [30]. Copyright license 3401820884539.
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initial oxygen stoichiometry of the HfO2 films influences their crystalline and electronic properties upon PDA. They have also studied the electrical properties of the 2 films and have observed a higher leakage current and a higher shift in the flat band volatge of the PDA low oxygen HfO2, implying that the initial stages of growth are decisive for the bonding between Hf-O and thereby the electrical properties.
Fig. 7 O-K edges from various grains in polycrystalline HfO2 after PDA. (a) low-oxygen content (O3 1s) HfO2 and (b) high-oxygen content (O3 5s) HfO2. The spectra (red dashed-dotted line) of (a) and (b) are dominant features in HfO2 films after PDA. Jang et al. J. App. Phys. 2011 [32]. Copyright license 3401820271310.
3. Doped Hafnia
3.1 Doping with +3 valence compounds
There are other factors that influence the electronic structure of HfO2, in addition to the processing parameter related ones viz., point defects, crystallinity and oxygen stoichiometry. These include doping which in fact, aims at increasing the dielectric constant of the material and reducing the leakage current either by creating oxygen vacancies in the monoclinic phase or stabilizing the cubic phase. However, addition of a dopant will also influence the electronic structure depending on the nature of the dopant by producing different bonding characteristics of the various cations to oxygen in doped HfO2. Wang et al. reported on the electronic structure of HfO2 on doping with Al and observed a variation in the imaginary part of the dielectric function for HfO2 in the VEEL spectrum, as a function of Al content (fig. 8(b)) [33]. Changes in the electronic structure of the 6 different Al doped HfO2 films grown by Pulsed Laser Deposition (PLD), are manifested right from small amounts of Al doping and no similarities between the VEELS of films with concentrations of Al= 0 and 1 exist. They have also studied the radial distribution function of Al doped HfO2 and suggest that Al-O bonds are formed at the detriment of Hf-O bonds and put forward a glassy HfO2-Al2O3 ad-mixture, seconded by the low loss EELS spectrum (fig. 8(b)). Changes in the onset of the plasmon peak with Al doping further suggest defect states within the band gap due to oxygen vacancies and cation interstitials. Since the conduction band minimum of Al2O3 is localized higher than that of HfO2, a net increase in the band gap is expected which in turn, is expected to produce an overall higher κ. Moreover, there are also defect states induced at the band edge of the Al doped HfO2 which can be explained in terms of the energy localization of the Hf d states or additional defect states due to oxygen vacancies and cation interstitials. Substitution of Al into a Hf site linked to an oxygen vacancy, would passivate oxygen vacancy related band gap states which in turn, is reflected in the onset of the plasmon peak between 3-5 eV of the VEELS (fig. 8(b), black arrow). Similarly, in the core loss O-K edged ELNES (fig. 8(a)), on addition of Al, the hybridization takes place between the O 2p states and Al 2p states. The distinct double peak feature indicates higher co-ordination between the
Fig. 6 valence EEL spectra from low-oxygen content HfO2 (O3 1s, solid line), high-oxygen content HfO2 (O3 5s, dashed-dotted line). Jang et al. J. App. Phys., 2011 [32]. Copyright license 3401820271310.
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Hf-O atoms and the Al = 0 film depicts a crystalline HfO2 phase. The peak separation is attributed to the crystal field splitting which occurs in all the polymorphs of HfO2. Nevertheless, further addition of Al destroys this local symmetry and the famous bi-peak feature seems difficult to discern. In the case of amorphous HfO2, the presence of the double peak feature persists, implying that some local co-ordination of Hf-O may nevertheless be possible despite loss of long range co-ordination. The peak broadening in the Al doped HfO2 spectra therefore, speaks for the local ordering and lack of long range ordering induced by point defects and a dominant amorphous character.
Fig. 8 (a) Electron energy-loss spectra showing the O K edges of the hafnium aluminate films with different aluminum concentrations (Al/Hf ratio of 0/1; 0.25; 0.67; 2; 5.8 and 1/0, respectively), (b) Electron energy-loss spectra showing the loss functions [(Im−1/ε)] of the hafnium aluminate films with different aluminum concentration. Wang et al.J. Appl. Phys. 2007 [33]. Copyright license 3401820055408. Not all dopants produce broad peaks in HfO2. In another study, Wang et al.[34] further compared the electronic structure of Al and Y doped HfO2 grown by PLD with nominal compositions of both dopant of ~10%, and observed that Y doping still produced comparatively sharper peaks than Al doping (fig. 9(a)) in the O-K edge ELNES. In fact, Y doped HfO2 continues to show the double peak fine structure of the O-K edge ELNES and bears a closer resemblance to the pure HfO2 O-K ELNES with the exception of slightly broader peaks in the former. The disappearance of the double peak structure in the Al doped HfO2 indicates once again loss of local co-ordination on doping with Al. Simulations were carried out using the FEFF program [35] to model the O-K edge ELNES, without considering the possible differences in the local chemical structure on substitution of Hf by Y (fig. 9(b)). Another simulation where simultaneously Y substitution and oxygen vacancies were considered, was carried out. They also simulated c-HfO2 O-K ELNES in their study, and the double peak appeared broader in the Y doped HfO2 than in the c-HfO2. This was assumed to be due to the smaller size of Y which could distort the O containing tetrahedron along with the presence of oxygen vacancies. Moreover, the intensity of the first peak is underestimated in the simulations. On the same lines, simulated EELS of Al doped HfO2 by substitution of Hf by Al up to the experimental concentration and by introducing oxygen vacancies was conducted (fig. 9(c)). Simulations were based on considering only the O4 co-ordination of the monoclinic structure but did not however reproduce the experimental spectra. These discrepancies could be due to the lack of two parameters for simulations: firstly, the O3 co-ordination and an amorphous model of HfO2. The differences in the simulated and experimental results further imply that substituting only Al on an Hf site is not enough. However, when an oxygen atom nearest neighbor to Al was removed the O-K edge ELNES changed drastically i.e. even though the double peak feature was subtly visible, it tended to resemble the experimental spectrum in fig. 9(c). In fact, their results shed light on the role of oxygen vacancies and their effect on the reduction of intensity of the first peak and broadening of the spectrum as a whole. Contributions from other defects also play a role and could result in further broadening of the spectrum.
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Fig. 9 (a) Electron energy loss spectra showing the O K-edge of the three thin films: pure, Y-incorporated and Al-incorporated HfO2. (b) Experimental and simulated oxygen K-edge of Y-incorporated HfO2. (c) Experimental and simulated oxygen K-edge of Al-incorporated HfO2. (Wang et al. J. App. Phys. 2008) [34]. Copyright license 3401821050030.
3.2 Doping with multivalent elements
Vanadium doped HfO2 has been studied in as early as 2001 by Turquat et al. [36];in fact, V has various oxidation states ranging from +2 to +5. In their study, the quantity of V at.% was varied between 7% and 30 % by using oxalic precursors under reductive (R), neutral (N) and oxidative (O) atmospheres, in the aim of obtaining the cubic phase of HfO2 and controlling the physical properties on doping. The O-K and V-L2,3 Edges ELNES spectra of the 3 vanadium samples; R30 and N20 which were pure cubic phases and O7 which was a mixture of monoclinic and HfV5O7 phases, were obtained (fig. 10(a)). The V-L2,3 edges appear at approximately 515 and 525 eV. From fig. 10(b), it is possible to fingerprint the oxidation state of Vanadium by comparing the intensities and energy positions of the L2,3 white lines along with the O-K ELNES. When HfO2 is doped with VOx, determining the oxidation with O-K ELNES is not straight forward as hybridization with Hf also plays a role. Therefore, small differences in the energy localization of the V-L2,3 edges, could also indicate mixed oxidation of V. They found that the oxidation state of vanadium was +3 and favored oxygen vacancies on substituting one Hf atom and consequently stabilizing the cubic phase. Moreover, the authors have measured the full width at half maximum of Vanadium white lines in samples R30 and N20 which were cubic phases of HfO2 and found them to be narrower than in the representative spectra (fig. 10(b)) implying that isolated V atoms as secondary phases could not be neglected. Furthermore, the electron lattice coupling can broaden the peaks especially if there are oxygen vacancies that increase the V-V distance for the final state 2p53dn+1. Nevertheless, for the samples grown under excess oxygen (O7) the tendency is different; the HfV5O7 secondary phase appears as big crystals and therefore, can be easily neglected by choosing a convenient probe size. Since the nominal composition of V in the sample is about 7%, the intensity of the EEL signal is not very pronounced but energy shifts are observed and when fingerprinted, correspond to the a higher valence such as +4 or +5 and therefore, being stoichiometric and over stoichiometric respectively, does not produce oxygen vacancies in order to stabilize the cubic phase. Like Vanadium, Tantalum also can adopt oxidation states ranging from +2 to +5. In fact TaN/HfO2 has been used as electrodes to replace poly Si/SiO2 stacks to increase performance. The effect of adding Ta to HfO2 was studied by Yang et al. where they deposited TaxHf1-xO2 and m-HfO2 layers successively and then annealed them.[37] They further carried out DFT calculations to understand the role that interstitial or substitutional Ta atoms play in HfO2. The high resolution TEM image (fig. 11(a)) consists of the Si substrate followed by the SiO2 interlayer, a partially crystallized TaHfOx layer and a well crystallized HfO2 layer. These features will therefore show a difference in the electronic structure of the Ta doped HfO2 and monoclinic HfO2. ELNES spectra were taken from 3 different zones: high Ta doped HfO2, low Ta doped HfO2 and m-HfO2 (fig. 11(b)). All spectra show the characteristic peaks of monoclinic HfO2 with a few differences. The spectrum Ta0.52Hf0.48O2 (fig. 11(b) middle) presents a broadened appearance which as seen from the HRTEM image, is due to its amorphous character creating locally distorted chemical structure. For the Ta0.1Hf0.9O2 ELNES, there appears to be differences in the onset of the doublet peaks named p1 and p2 in this study. The shoulder just before the p2 peak is in fact deduced to be the hybridization of O 2p with Ta 5d and Hf 5d states. From DFT calculations, authors further obtained the formation energies of substitutional Ta and two interstitial Ta defects to be 3.93, 7.72, and 7.28eV and have concluded that Ta in general substitutes Hf but the simultaneous presence of interstitial
Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.)__________________________________________________________________
and substitutional Ta cannot be neglected. In their study, they consider Ta to be pentavalent and of the same size as Hf, thereby suppressing differences in the O bond lengths. However, Ta acts as a donor and therefore gives the band structure of HfO2 a metallic character which nevertheless can be modified through the presence of defects such as dislocations. This implies that Oxygen vacancies are highly improbable in this Ta doped HfO2 study and the stabilization of the cubic phase is therefore difficult.
Fig. 10 (a) Comparison between the V 2p EELS spectrum of R30, N20, and 07. (b) Comparison between the V 2p EELS spectrum of vanadium oxides. (Turquat et al., Int. J. Inorg. Mater. 2001) [36]. Copyright license 3407490664269.
Fig. 11 (a) High-resolution TEM image of m-HfO2 layer deposited on the top of Ta 0.52 Hf0.48O2 film, and (b) O K-ELNES recorded on m-HfO2, Ta0.1Hf0.9O2, and Ta0.52Hf0.48O2 layer. Yang et al. J. Appl. Phys. 2011 [37]. Copyright license 3401821115347.
3.3 Diffusion of dopants with +2 oxidation states studied with EELS imaging
Doping in fact is aimed at increasing the flat band voltage among other physical properties of HfO2. However, these metallic dopants undergo diffusion during growth and annealing. Moreover, the diffusion dynamics of a dopant vary depending upon the species. In this section, understanding the diffusion behavior of these metallic dopants by EFTEM and SI using the EEL signal will be discussed through various works. Rauwel et al. [38] have studied the effect of Mg dopant on the physical properties of HfO2 along with using the O-K, Mg-K edges to understand the diffusion of Mg into HfO2. In fact, a thin layer of Mg was deposited as an interlayer between HfO2 and the Si substrate. The aim of using Mg was to get rid of the SiO2 interlayer and also decrease the EOT by its addition. Uniform distribution of Mg was assured through a rapid thermal annealing (RTA) of the sample. The high resolution images presented in fig. 12, illustrate the as-grown Mg interlayer of 2 nm (fig. 12(a)) which on annealing disappears (fig. 12(b)) as a result of diffusion of Mg into HfO2. The diffusion of Mg has 3 effects: it suppresses the SiO2 interlayer growth, recrystallizes the partially crystallized sample and stabilizes the cubic phase as shown in fig. 12(b). The cubic phase is in fact stabilized by the oxygen vacancies created by Mg2+ oxidation. Furthermore, the Mg-K edge was used to map out the Mg present in the film and shows a uniform distribution upon annealing (fig. 12(f)). The study does not propose any error bars necessary in the case of elemental edge mapping by EELS and drift of the sample with recording times of 8s is likely. Other elements display less uniform doping such as Ba. Bruley et al. have studied the effect of Ba doping on HfO2
[39]. This was achieved by depositing a fine layer of Ba on the surface of HfO2 and then annealing. Diffractograms gave higher temperature phases such as cubic or tetragonal due to the Ba2+ oxidation state. In this study, EELS was not used to study the electronic structure but rather the diffusion of Ba on annealing and showed that some Ba atoms are
Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.)__________________________________________________________________
incorporated into the film while others mostly gather at the interlayer and also diffuse across the interlayer into the Si substrate. Moreover, Ba has a tendency to damage moderately the microstructure the HfO2 film.
Fig. 12 Comparison of two HRTEM micrographs obtained from cross-sections of thin HfO2 films deposited on Mg interlayer: (a) As deposited and (b) after RTA treatment at 800°C for 1 min under N2. The metallic intermediate interlayer decreases from 2 nm to ~0.3nm thick. The inset is a fast Fourier transform of the encircled grain presenting the HfO2 cubic phase and oriented along the <001> zone axis. EFTEM mapping of the annealed film in (b), (c) Zero loss image, (d) Si plasmon map, (e) SiO2 plasmon map and (f) Mg–K edge map. Rauwel et al. Thin Solid Film 2012 [38]. Copyright license 3401810444504.
Fig. 13 EELS profile showing Ba diffusion through HfO2 and into SiO2 IL (b) HAADF image before (top) and after (bottom) line profile. Shoulder on line profile suggests modest damage to interlayer. (Bruley et al. Microsco. Microanal. 2010) [39]. Copyright license 3401810244296.
A different tendency is shown by Co that aggregates on the surface layer of HfO2 as studied by Rao et al. [40] HfO2 thin films were grown using PLD on Yttrium Stabilized Zirconia substrates and doped with Co during growth itself. EELS line scans across the thickness of the HfO2:Co film gave Co, L2,3 white lines at the top 10nm of the film indicating the tendency of Co to aggregate at the surface. The incorporation of Co into the HfO2 lattice was clear from the XRD pattern which indicated a monoclinic phase of HfO2, along with RBS measurements. This indicates that the cobalt tends to segregate out after reaching a solubility limit. However the low concentration of Co within the sample may not simplify its detection by EELS. The white lines intensity and positions indicated a +2 valence for cobalt. The authors however claim that Co 3+ valence was also detected from XPS studies. No stabilization of the cubic phase was observed in this study despite using a +2/+3 valence dopant, probably due to lack of annealing.
Fig. 14 (a) High resolution Z-contrast STEM image of Co-doped hafnia film showing epitaxial growth on YSZ, (b) the EELS line scans shown bring out the presence of Co-rich surface layer in top 6 nm. No Co signal detected in the film, (c) establishes more clearly the appearance of a thin layer on the top which is attributed to Co-rich surface layer (Rao et al. Appl. Phys. Lett. 2006) [40]. Copyright license 3401821172694.
Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.)__________________________________________________________________
In advanced Si based CMOS devices where the thickness of HfO2 can be as small as 1nm, understanding its electronic structure which differs from its bulk counterpart is indispensable. We have seen that the bonding between Hf and O can modify the physical properties viz., leakage current, mobility, flat band voltage and dielectric losses, of the HfO2 gate oxide. EELS is a powerful technique to study the unoccupied states above the Fermi level in HfO2 with sub-Ångstrom spatial resolution, possible today with Cs correction. Since ELNES results when the incoming electron excites a core level electron into an unoccupied state above the Fermi level [21], it therefore provides us with information on the site and symmetry projected density of states. The low-loss region or VEELS (0–100 eV) on the other hand, reflects collective valence electron excitations which include plasmons and single valence electron excitations into unoccupied states in the conduction band [21]. Nanostructuring of HfO2, furthermore, brings about changes in the volume plasmon and intensifies certain surface plasmons. The crystal and electronic structures as well as the physical properties of HfO2 vary according to growth conditions and PDA treatments. In fact through PDA, phase transformations from monoclinic to tetragonal to cubic as the temperature increases, can occur. All the same, doping with divalent and trivalent ions stabilizes the higher symmetry phases at low temperatures by inducing oxygen vacancies. It is also possible to stabilize the higher temperature phases by oxygen vacancies alone i.e. without doping and by employing reductive solvents or precursors. Considering the purely ionic model, Hf 5d bands are empty, and the O 2p bands are fully occupied, due to the 5 d0 electronic configuration. Electronic structure calculations of partial densities of states also show that the upper valence band is formed by the O 2p states and the lowest conduction band is formed mainly by the cation d-states [41-43]. At higher energies, bands are formed from O 2p mixed with metal (n+ 1) sp states (n = 5 for HfO2). The O-K edge ELNES provides us with this bi-peak feature that is consistently present in HfO2 irrespective of the crystallinity. They correspond to the hybridization of the Hf 5d and O 2p states, implying that even in amorphous HfO2 short range Hf-O co-ordination exists. The O-K ELNES is also sensitive to processing parameters of HfO2, presence of dopants in HfO2 and oxygen vacancies which affect the shape of the O-K ELNES, onsets and energy localizations of the peaks. Diffusion dynamics of dopants is an important aspect to be considered as not all dopants diffuse in a uniform manner. Excess Ba has a tendency to diffuse into the substrate, excess Co is concentrated on the surface of HfO2; while as, Mg has a tendency to diffuse uniformly throughout the film. Since EELS is a technique that analyses the local electronic structure whose area is limited to the size of the probe, understanding the diffusion of these dopants is possible at the nanometer scale and variations in the electronic structure as a function of dopant concentration can be studied through EELS along with EFTEM and SI.
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